1. Field of the Invention
The present invention relates to a fuel cell assembly with improved connecting structure, and in particular to a fuel cell assembly having conductive nets attached thereto to transfer electrons.
2. Description of the Related Art
Fuel cells (FC) directly convert chemical energy in hydrogen and oxygen to electricity. Compared to conventional power generation devices, fuel cells produce less pollution and noise, and have higher energy density and energy conversion efficiency. Fuel cells provide clean energy, and can be used in portable electronic devices, transportation, military equipment, power generating systems, or the space industry, among many other applications.
Different fuel cells use different operating principles. Direct methanol fuel cells (DMFC), for example, use, on the anode side, methanol solution to precede oxidation, producing protons (H+), electrons (e−) and carbon dioxide (CO2). The resulting hydrogen ions diffuse through an electrolyte toward the opposing cathode. Meanwhile, oxygen is fed to the cathode. As the proton, electrons (e−) and oxygen are combined on the cathode side, water is formed. The voltage between electrodes drives electrons from the anode to the cathode sides via external loading. The net result is that the DMFC uses methanol to produce electricity, with water and carbon dioxide as by-products.
The output voltage of a single cell is too low to drive any electronic device. Several fuel cells must thus be connected in series as a fuel cell stack to provide sufficient output voltage. When connecting fuel cells, transmission of generated voltage from one fuel cell to another, especially electrons from the anode of one fuel cell to the cathode of another, must be accomplished.
In FIG. 1, a conventional fuel cell 10 includes an anode 12, a proton exchange membrane (PEM) 11 and a cathode 13, forming a membrane electrode assembly (MEA) Two gas-diffusing layers 14 thereon are formed by carbon cloth or carbon paper. Moreover, the catalyst, the MEA, the gas-diffusing layers 14, the bipolar plate 15 and the end plates 17 of the conventional fuel cell are assembled by screws 16 to provide proper electrical connection conductivity and fuel supply.
However, problems exist with the above conventional connection structure. Force provided by the screws is not uniform, increasing the resistance of the fuel cell assembly or blocking fuel passing through the grooves of the end plates 17 and the bipolar plate 15. Additionally, this structure requires extra space allowance for screws, which reduces the assembling density. For flat fuel cell assembly in a portable device, the assembling density is reduced by 30%˜50% and cannot satisfy the requirement for high power density.
In conventional stacked fuel cell assembly, the bipolar plates connect the anode and the cathode of each two neighboring fuel cells and separate the required fuel as shown in FIG. 1. However, the conventional stacked fuel cells are not suited to use with portable devices, such that a new plane-arranged fuel cell assembly is needed.
U.S. Pat. No. 6,277,658 to Pratt et al. describes a method of using two plastic frames with current collectors to sandwich an MEA. However, the current collectors are metal nets, and the force exerted by the plastic frames may be not uniform, such that the metal nets do not closely contact the anodes and the cathodes of the MEA, increasing resistance and reducing output voltage of the fuel cell assembly.
Many structures and connecting methods for flat fuel cell assemblies have been disclosed, such as Rongzhong et al. (J. of Power Source, 93, 2001, 25-31), A. Heinzel et al. (Electrochemica Acta, 43, 1998, 3817-3820), S. J. Lee et al. (J. of power Source, 112, 2002, 410-418).
U.S. Pat. No. 6,277,658 to Cisar et al. also describes an improved fuel cell design for use at low pressure. The invention has an electrically conductive porous material directly bound to conductive nets comprising an insulating binding component and an electrically conductive component, such as an intimate mixture of a powder and loose fibers, to form gas-diffusing layers. These gas-diffusing layers are then bound to the MEA by heat pressing and the fuel cells are connected in series.
Problems continue to exist with the method provided by Cisar et al, since the electrically conductive porous material must be applied to different areas on opposite surfaces of a conductive net by conventional printing, uniformity of the electrically conductive porous material is hard to control. Moreover, holes in the gas-diffusing layer formed by the conventional printing process are smaller than holes in the gas-diffusing layer formed by a carbon cloth. Thus, the thickness of the printed gas-diffusing layer must be reduced or precisely controlled, but conductivity and stability of the fuel cell assembly will deteriorate.